1. Field of the Invention
The present invention relates to a photovoltaic device. More specifically, the present invention relates to a photovoltaic device such as a solar cell employing amorphous silicon.
2. Description of the Prior Art
Solar cells employing various kinds of semiconductor materials have been heretofore proposed and put into practical use. Such solar cells utilize a single crystal or polycrystal semiconductor material such as Si, GaAs, CdS, CdTe or the like and employ a p-n junction, a heterojunction or a Schottky barrier. Recently it was observed that amorphous silicon obtained by glow discharge decomposition of silane (SiH.sub.4) makes it possible to control the conductivity type by doping of a substitutional impurity. Ever since this observation attention has been focused on this process as a process for inexpensively manufacturing solar cells which meet the demand of development of a substitute energy source. Several disclosures have been made of an amorphous solar cell manufactured using a glow discharge, for example, in U.S. Pat. No. 4,064,521 issued Dec. 20, 1977 to Carlson. These disclosures have employed a junction similar to that of conventional solar cells.
However, in the above-described solar cells, an open-circuit voltage Voc withdrawn from a solar cell unit including a single p-n junction is determined by the energy gap of the semiconductor material used in forming the junction. For example, in the case of a silicon solar cell, a voltage of approximately 0.6 V is obtained and in the case of a GaAs solar cell, a voltage of approximately 0.8 V is obtained, while the open-circuit voltage Voc is no higher than 1 V. However, a higher open circuit voltage output is often required in actual applications of such cells. A conventional approach, which was employed for generating a higher voltage output, was that a plurality of solar cell units were fabricated and then externally connected in series. Although various proposals concerning a means for connecting solar cell units in series have been made, disadvantages have been encountered in that the package cost is high in manufacturing facilities and process and the cells are large.